Reduce probability of initial prompt tasks (#1764)

The weight of initial prompt tasks for random task selection is reduced
to 0.01 which makes it very unlikely to be selected as long as other
tasks such as reply or ranking are available.

Closes: #1758

backend/oasst_backend/tree_manager.py

@@ -193,7 +193,7 @@ def _random_task_selection(
             task_weights[TaskType.REPLY.value] = 2
 
         if num_missing_prompts > 0:
-            task_weights[TaskType.PROMPT.value] = 1
+            task_weights[TaskType.PROMPT.value] = 0.01
 
         task_weights = np.array(task_weights)
         weight_sum = task_weights.sum()